The intent of this proposal is to determine the distribution, origin and physiological effect(s) of two putative neuromodulators, the indoleamine serotonin and the peptide enkephalin, on Purkinje cells in the cat's cerebellum. The Specific Aims of this proposal are: 1) To determine the lobular and laminar distribution of serotonin and enkephalin in the cat's cerebellum using the peroxidase anti-peroxidation technique. 2) To localize the cells of origin of serotoninergic and enkephalinergic varicosities present within the cerebellum. This will be accomplished by using a double label technique which combines retrograde transport of horseradish peroxidase (HRP) with the PAP technique. 3) To analyze the physiological effects of serotonin and enkephalin on Purkinje cells. Various putative neuroactive substances (e.g. serotonin, enkephalin, glutamate, GABA) will be iontophoresed from multibarrel extracellular recording electrodes and their on Purkinje cell activity recorded. In some experiments an intracellular electrode will be combined with the drug electrode. The location of each unit will be marked either with Fast Green (extracellular recording) or HRP (intracellular recording). The tissue will be processed with the PAP technique to verify the presence of serotoninergic and enkephalinergic varicosities in areas from which physiological data are obtained. 4) To establish the relationship of serotoninergic and enkephalinergic varicosities to other axonal elements within the cat's cerebellum. The spatial relationship of immunoreactive varicosities to Purkinje cells intracellularly injected with HRP will be analyzed. In addition, the cytological characteristics or terminals containing serotonin and enkephalin as well as their synaptic relationships will be determined. The role of these chemically defined systems has not been incorporated into existing theories of cerebellar function as little is known about their effect(s) on specific populations of cortical neurons. It has been proposed that neuromodulators play a role in regulating neuronal activity and that they function to bias or fine tune neural circuits which, ultimately, influence on-going- movements. Thus, data from the proposed studies, when incorporated into our existing concepts of cerebellar physiology, will further extend our knowledge of how individual circuits in the cerebellar cortex contribute to the control and co-ordination of movement.